The present invention relates generally to instrumentation for, and applications of, vacuum ultraviolet reflectance spectroscopy. In one embodiment, the invention can be used to provide semiconductor manufacturers with high throughput, non-contact metrology capabilities for use in process control during the manufacturing of leading edge semiconductor devices. Additionally, the present invention is sufficiently compact so as to facilitate its use in integrated (in-line) process control applications.
The semiconductor industry is currently developing processing technologies to enable manufacturing of devices comprised of thinner layers and possessing smaller feature sizes than at any time in history. To make possible these advances, supporting metrology techniques must be developed as current systems lack the sensitivity required to detect subtle changes in processing conditions.
Optical metrology instruments have long been used in semiconductor processing applications because they are typically non-contact, non-destructive and enable high measurement throughput. The vast majority of these instruments employ ellipsometry and/or reflectometry methods to characterize materials. Traditionally these instruments have been used to measure film thicknesses, optical properties and other material characteristics like composition, porosity, and roughness. More recently, interest has arisen in the extension of said instrumentation to characterize critical dimensions of device features through use of scatterometry modeling techniques.
Virtually all current ellipsometry and reflectometry metrology instruments operate in some portion of the spectral region between deep ultraviolet (˜200 nm) and near-infrared (˜1000 nm) wavelengths. Unfortunately, as semiconductor processing technologies progress, and as device geometries shrink, the sensitivity of such instruments to changes in processing conditions is reduced. Without sensitive, accurate, and repeatable feedback from metrology instrumentation, semiconductor manufactures are unable to adequately control process equipment and hence, achieve high yields. In short, conventional optical instrumentation is reaching limitations that make it unsuitable for future technologies.
A select number of companies have recognized the evolving need for vacuum ultra-violet (VUV) (generally wavelengths less than 190 nm) optical metrology equipment and have manufactured commercial products targeted to address this requirement. Examples include Sentech Instruments GmbH of Germany, J.A. Woollam Co., Inc. of the U.S. and Sopra Inc. of France. All of these companies, however, have designed instruments that rely on ellipsometric techniques, which by their very nature require complicated hardware and control systems. Such devices are typically slow and not capable of sustaining the high level measurement throughput required in semiconductor manufacturing environments. In addition, such instruments generally employ numerous transmissive polarizing elements which effectively limit the shortest wavelength photons that can be employed. As a result, current VUV ellipsometric metrology systems can only operate at wavelengths longer than about 140 nm.
Also, the prior art U.S. Pat. No. 6,414,302 (Freeouf) describes the benefits of performing high energy specular bidirectional ellipsometric measurements in a geometry where the entire light path is maintained in a controlled environment. While this approach does attempt to make use of the improved signal distinguishing ability imparted by the use of higher energy photons, it also suffers from the aforementioned complications associated with the employment of complicated ellipsometric techniques. In addition, this design requires the ambient to be strictly and reproducibly controlled both during the actual measurement, as well as between series of measurements in order to obtain accurate and reproducible results. That is, to minimize the uncertainty in the measured data resulting from environmental influences it is necessary to ensure not only that appropriate environmental conditions are realized, but also that the same appropriate conditions be reproducibly achieved and maintained during each and every measurement. If the conditions change, instrument repeatability and stability will be adversely affected.
McAninch, in U.S. patent application 20020149774, discloses a purge system for an optical metrology tool which does not require placing the sample in a controlled environment. Reproducible steady-state conditions would be difficult to achieve using such an arrangement as flow characteristics could be expected to change considerably depending on the placement of the sample during measurement. Other factors like sample size and the presence of patterned structures on the surface of the sample could also be expected to influence flow characteristics. Additionally, this disclosure makes no mention of how data referencing could be incorporated into the design.
Other companies such as Acton Research Corporation and McPherson, Inc., both of the U.S., have also developed optical instruments for performing spectral measurements of reflectance and/or transmittance in the VUV region. Generally speaking, these systems employ step and scan technology, whereby a spectrum is recorded through use of a single element detector in combination with the scanning of a diffraction grating through a range of angles. As such, these systems are time consuming and not well-suited to the needs of semiconductor manufacturers.
In order to achieve highly repeatable results with a reflectometer it is necessary to provide a means by which reflectance data can be referenced or compared to a relative standard. In this manner changes in the system that occur between an initial time when the system is first calibrated and a later time when a sample measurement is performed can be properly accounted for.
Numerous referencing techniques exist in the prior art, generally however these methods are either time consuming, and involve mechanically positioning a reference sample into and out of the sample location, or employ separate detection components (i.e. diffraction elements and detectors) for sample and reference measurements which can lead to inaccurate results. For example, one approach utilizing separate detection components to reference reflectometer data is to employ a “dual beam” configuration. In this arrangement broad-band light is typically dispersed using a spectrometer in order to create a monochromatic exit beam. This beam is split into two parts; the sample beam and the reference beam. The sample beam is reflected from the sample and recorded by a sample detector, while the reference beam bypasses the sample and is recorded by a reference detector. The approach is time-consuming since it requires scanning of the dispersion grating and also suffers from the detector issues described earlier.
Hence, it would be desirable for an optical measurement tool to incorporate a highly accurate means of referencing which was rapid and compact so as to facilitate its use in in-line metrology applications.
As device geometries shrink metrology instruments are expected to perform measurements on smaller and smaller regions of samples. It would be highly advantageous if a metrology tool were able to simultaneously perform measurements on a number of such sites within a localized region of a sample in order to obtain greater information without the increased time typically associated with the re-positioning and re-alignment of the sample.
As none of the currently available optical metrology technologies overcome the difficulties associated with collecting accurate and repeatable optical data in the VUV region, it follows that there would be great benefit in designing such an instrument.